The present invention relates to composite structural products, and more particularly relates to a system and a method for manufacturing composite structural products out of reinforcing materials such as wood, plastic, ceramics, cloth, glass and concrete wherein an adhesive is used to bond together pieces, fibers or fragments of the reinforcing materials.
Many structural products are made by bonding together pieces or fragments of materials to make a larger piece. In the wood bonding industry, a common wood-composite-board manufacturing process includes drying wood chips or veneers, applying adhesives and forming the final product in a cold press and/or a hot press. Typically, wood pieces or fragments and heat curable adhesive are combined and heat and pressure is applied to the combination to set the adhesive.
In many processes both cold presses and hot presses are used. First, cold presses are used to prepress the material in a manner sufficient for handling while hot presses are then used to set the adhesive. Because cold pressing provides sufficient consolidation or bonding to permit handling of panels (plywood and beams), prepressed laminates or consolidated panels may be stored for a few minutes to hours before they are finally subjected to high heat and pressure to set the adhesives.
Synthetic resins that are predominately used in the manufacture of plywood, particle board and fiberboard bonding, for example, are set in hot presses at temperatures of between 110 to 205xc2x0 C. for 3 to 8 minutes depending on the type of wood and adhesive. Electric coils, high frequency current, steam and oil or gas have all been used to heat the press platens or materials to be bonded. Pressure is then applied to a panel by engaging a series of hydraulic pistons and rams or other means of applying pressure. Also, high frequency electric fields or radio waves have been used to excite particles in the adhesive, thus causing heat to be generated within the adhesive for curing. These systems have extensive equipment requirements and thus are expensive and cumbersome.
Curing or setting the adhesive in the manner described above, requires that the wood or other material have a low moisture content, typically between two (2) and eight (8) percent. However, because most wood is reduced to pieces or fragments before it has been dried, its moisture content is between thirty (30) and sixty (60) percent. In the wood products industry, wood having a moisture content as described is referred to as green, or freshly cut wood. Traditionally, the reason the starting materials prior to pressing are between 2-8% moisture is to speed up press times. For example, heat barriers form in moist wood which in turn slows heat transfer to the core region during the pressing step, thus slowing the pressing process.
In order to remove the excess moisture from the green wood, the wood is subjected to temperatures as high as 870xc2x0 C. in drying kilns which are commonly fueled by natural gas, propane oil or incinerated scrap wood and wood residues. Depending upon the initial moisture of the green wood, the drying time can extend several hours. Moreover, as the wood nears the target moisture content, the drying process becomes more energy and time consumptive. Also, due to the length of time the wood spends in a dryer during a typical processing operation, dryer capacity becomes a limiting factor in efficiency of the overall wood-bonding process.
As mentioned above, hot pressing is used to set adhesive to bond fibers or filler together to form a larger piece. Hot press platens are forced against the longitudinal sides of the board to be formed. The applied heat is therefore transferred from outside the board to inside the board. In manufacturing wood and other composite products, because of the low thermal transfer properties of wood and other composite elements, the composite boards generally have to be heated for two minutes after the center layer of the board reaches 100-180xc2x0 C. since the heat transferred from the hot press platens to the inner regions of composite board materials is slow. The low thermal transfer properties thus cause the outer portions of the composite boards to be frequently overheated while the inner layers may be underheated. Therefore, in the use of hot press platens, not only is energy wasted by unnecessary heating thermally insulating substrates and by the loss of heat to the ambient environment, but the outer portions of the composite board can be damaged since they are subjected to excessive heat which may lead to warpage.
As an example, the manufacture of plywood illustrates the inefficiency of existing manufacturing means. Layered veneers are pressed between heated platens for three to eight minutes. When the veneers are heated to requisite temperatures of 145 to 205xc2x0 C., the substrates partially degrade in the heat. Furthermore, loading and curing times of hot presses cause major bottlenecks in plywood manufacture.
The present invention provides a system and a method which avoids the necessity of nearly completely drying out the wood pieces or fibers prior to their combination with adhesives. Also avoided may be the necessity of using hot press platens to set the adhesives for bonding together wood pieces or fibers. In regard to high frequency bonding equipment, the complicated and cumbersome equipment requirements are avoided by using a system method of the present invention. By causing otherwise electrically non-conductive pieces or fragments of wood, including green wood, to become temporarily electrically conductive in the manner described below, these pieces or fragments can be used as part of an efficient, economical and rapid process of heating and thereby setting or curing the cooperating adhesives. More specifically, an electric current is caused to flow through the temporarily conductive fragments of wood to generate heat for setting or curing the adhesive. This, in turn will result in a more efficient method and system of making composite structural products where the capacity of a new or existing processing plant can be increased substantially while processing equipment requirements can be reduced considerably.
The present invention also includes a product made by the steps of providing electrically non-conductive reinforcing pieces, fibers or fragments of wood, in their natural, in a treated or a synesthetic form, causing the electrically non-conductive pieces or fragments to be temporarily electrically conductive, combining adhesive which sets when sufficient heat is applied thereto with the temporarily electrically conductive pieces or fragments, and applying voltage across the temporarily electrically conductive pieces or fragments and adhesive combination so as to produce an electric current through the temporarily electrically conductive pieces or fragments which, in turn, generates heat within the temporarily electrically conductive pieces or fragments and adhesive combination.